Do not process null network packets on all clients

Packets received from the "null" networks are packets received from
tethering downstream interfaces. They should not be processed by all
MdnsServiceTypeClients; instead they should be processed by
the MdnsServiceTypeClient for the null network.

Bug: 283708537
Test: atest
Change-Id: Ifef59eca7a24bdfe3650067445d2565869dfa852